1. Field of the Invention
This application relates generally to a method and apparatus for controlling operation of a gas turbine for electric power generation, and more specifically to a backup method and apparatus for overriding control of a gas turbine to reduce damage to the gas turbine when an operational irregularity occurs.
2. Description of Related Art
Conventional backup control curves have been generated for gas turbines to reduce operation of the gas turbines in the event that an operational parameter falls outside a range of allowable values. The reduced operation of the gas turbine is designed to lower the output of the gas turbine from its standard output under normal operating conditions to avoid damage to the gas turbine.
One such operational parameter, the exhaust temperature of the gas turbine, can be predicted based on the power output by the gas turbine. Thus, the power output by the gas turbine can be correlated to a predicted exhaust temperature. When the power output by the gas turbine exceeds an upper limit the exhaust temperature is determined to be above a satisfactory operational temperature above which the gas turbine becomes prone to being damaged or prone to inefficient operation. Under such conditions the controller initiates a backup control routine to reduce the output of the gas turbine to minimize the likelihood that the gas turbine will be damaged.
For example, FIG. 1 shows a conventional backup curve 2 and a control curve 4 presented on a grid of exhaust temperature of the gas turbine versus ambient pressure. The control curve 4 is generated for the gas turbine and programmed into a controller to establish a target exhaust temperature of the gas turbine under normal operating conditions to achieve a desired output. The conventional backup curve 2, on the other hand, is also generated for the gas turbine and programmed into the controller to establish an upper limit of the exhaust temperature instead of a target exhaust temperature. If the exhaust temperature is determined to reach or exceed this upper limit a backup control routine is executed by the controller to limit operation of the gas turbine and bring the exhaust temperature back down to a suitably-low temperature.
As can be seen in the example shown in FIG. 1, the conventional backup curve 2 exhibits a significant pressure dependence, while the control curve 4 remains relatively constant over the range of ambient pressures shown. As the pressure in the ambient environment of the gas turbine increases and exceeds a reference value of ambient pressure, the conventional backup curve 2 intersects and extends below the control curve 4 just below a pressure of 14.8 psia. At pressures greater than the 14.8 psia ambient pressure in FIG. 1 the exhaust temperature requested according to the conventional backup curve 2 is less than the target exhaust temperature requested according to the control curve 4. Thus, at elevated pressures the conventional backup curve 2 will call for lower exhaust temperatures than the control curve 4, meaning that gas turbine will provide a lower power output than afforded by the control curve 4 at these elevated pressures. In other words, the controller provided to the gas turbine will unnecessarily control operation of the gas turbine according to the conventional backup curve 2 to limit the output of the gas turbine under elevated pressures when operation of the gas turbine according to the control curve 4 is otherwise appropriate.
A method and apparatus for executing a backup control routine that accounts for changes in pressure that may occur in an ambient environment of a gas turbine would provide benefit over the known methods and systems.